Mass spectrometry is well recognized for use in chemical analysis due to the high resolution measurements that can be realized and because a mass spectrometer measures a fundamental property of chemicals that are introduced to the instrument. Other forms of chemical analysis instrumentation such as ion mobility spectrometers, surface acoustic wave devices, electrochemical cells, and similar instruments measure the constituents of a sample by inferring their presence from measurements of related phenomena such as resonant frequency changes, voltage changes, and drift time measurements.
Mass spectrometers operate at pressures well below that of these other instruments. Mass spectrometers typically operate at pressures of 10−6-10−3 Torr, while other analytical instruments typically operate at approximately one atmosphere of pressure. It should be noted however, operating pressures for instruments that are different than those referenced here may be selected based on the specific design of a particular instrument without changing the nature of the implementations disclosed herein.
Because mass spectrometers operate at pressures well below that of atmospheric pressure, there will be fewer molecules present per unit volume in the instrument than for those instruments that operate at higher pressures. This is well described by the Ideal Gas Law:pV=nRT
where p is the pressure inside the analysis chamber of an instrument, V is the volume of the analysis chamber, n is the number of molecules present, R is a constant equal to 8.314 J mol−1 K−1, and T is the temperature of the sample.
For many applications, it is desirable to reduce the size of chemical analysis instruments. For example, it may be desirable for screeners in airports to carry an instrument through the facility that can detect the presence of explosives by analyzing the air around suspicious persons or objects and looking for traces of explosive material. Another example is that it may be desirable for first responders to carry an instrument to the scene of a fire or chemical emergency to gain foreknowledge of which chemicals are present. A further example is that it may be desirable for a health care professional to have a portable instrument that can be carried to a patient's bedside to analyze the patient's breath for chemicals that can indicate disease. It should be noted however that these examples are merely provided as illustration of the need for miniaturized instruments.
As mass spectrometers are decreased in size to that which enables easy portability, the volume of the instrument is decreased. Because mass spectrometers typically use lower operating pressure than other chemical analysis instruments, and as the mass spectrometer is decreased in size for ease of portability, the number of molecules present in the instrument during analysis is significantly reduced. This is illustrated by the Ideal Gas Law noted above by decreasing both p and V; as a result, the number of molecules present, n, is reduced accordingly.
The effect of reducing the detection volume of the instrument is to reduce the sensitivity of the instrument, where the sensitivity is the minimum external concentration of sample that can be measured by the instrument. For example, a mass spectrometer operating at 10−3 Torr, with an analysis chamber volume of 1 mm3, operating at 25° C. will have 32.3×109 molecules present. A corresponding instrument that operates at atmospheric pressure (760 Torr) will have 24.6×1015 molecules present. A corresponding instrument that operates at 10−3 Torr but has an analysis chamber that is 1 cm3 will have 32×1012 molecules present. Note that these calculations are provided to illustrate that miniaturizing instruments that operate at lowered pressured can have a significantly lower number of molecules available for analysis. Instruments that operate at other pressures and/or have analysis chambers of different volumes can be analyzed and similar calculations performed.
If a sample is introduced to a miniature mass spectrometer, the chance of detecting the presence of a chemical of interest present in that sample is thus significantly reduced. Typical field portable mass spectrometers are capable of detecting the presence of chemicals in an air sample introduced to the instrument down to approximately 1 ppm (parts per million). Techniques are available to those skilled in the art to improve the sensitivity of the instrument. For example, by coupling a mass spectrometer with a gas chromatograph, using special thermal desorption probes, and repeating the analysis multiple times. It should be noted that other examples exist and these are provided only for illustration purposes. The problem with the use of these techniques is that the time required to perform an analysis is significantly increased, typically from several seconds to several minutes; or in the case of a gas chromatograph coupled to a mass spectrometer, up to typically 30 minutes.
To be effective, portable instruments must be capable of detecting chemicals present at or below 1 ppb (parts per billion). For example, Table 1 shows the Immediate Danger to Life and Health (IDLH) values for several common Chemical Warfare Agents (CWAs) (adapted from Sun, Y. and Ong, K, Detection Technologies for Chemical Warfare Agents and Toxic Vapors, CRC Press, 2005).
As can be seen from examination of this table, some of the common agents are dangerous at concentrations down to 2 ppb; hence, instruments must be able to detect below this level.
TABLE 1CWACASIDLH (ppm)GA71-86-60.030GB107-44-80.030GD96-64-00.008GF329-99-70.030VX50782-69-90.002
Also, for a mass spectrometer to be able to detect a chemical, it is introduced to the instrument in a gaseous form. Consider that many explosives have very low volatility indices and as such, emit a very low amount of vapor into the surrounding air.
As a result, for a portable instrument to be able to detect the presence of explosives simply by analyzing the air in the proximity of the instrument, it must be able to detect concentrations to extremely low levels, ideally parts per trillion (ppt).
To facilitate this low concentration detection, some systems include a chemical pre-concentrator to increase the apparent concentration of samples being introduced to the chemical analyzer. For example, the apparent concentration of a sample introduced into an analyzer can be increased by using a membrane between the sample inlet and the chemical analyzer to remove or block certain species, while allowing target species to flow into the analyzer. While membrane inlets have been proven effective in commercial applications, they are typically limited to small concentration gains (<100) and are selective in the types of materials that are allowed through the membrane. An alternative approach is to use solid sorbent tubes to trap the species of interest. Conventional sorbent tubes are typically composed of a metal or glass tube packed with glass fibers or beads coated with or comprised of absorptive material, solid absorbent (e.g., calcium chloride, silica gel), or a variety of sorbent materials suited for the particular application. It should be noted that the terms absorption (implying an interaction of the analyte with the bulk material) and adsorption (implying an interaction with the surface of a material) are both used interchangeably. The specific mechanism of collecting analyte is material dependant and all forms of collection are covered by the scope of this disclosure. The tubing is typically wrapped in Nichrome wire which heats the tubing when an electrical current is passed through it. During the collection phase, a sample is passed (e.g., by carrier gas, or liquid) through the tube while the sorbent material adsorbs the analyte. These sorbents are then heated, releasing the analyte into the analyzer in a much shorter time than they were absorbed, thus increasing the concentration “seen” by the chemical analyzer.
Indirectly heating the sorbent material often results in various inefficiencies. For example, the sorbent material typically provides poor heat conduction paths, thus hindering the heat flow to the interior of the sorbent material. Further, additional power and time is typically required to compensate for the loss of heat into the surroundings. Desorption time is also important from a performance point of view since the concentration gain is inversely proportional to the time required for desorption. In addition, the sorbent material often impedes the passage of the carrier gas during sampling and desorption. Still further, while large gains in concentration are possible, conventional assemblies may have other drawbacks, including, for example, 1) there can be a substantial amount of time and power required to adsorb & desorb sufficient material; 2) the various locations on the sorbent material are not heated simultaneously thus releasing the chemical at different times and hence, reducing the apparent concentration seen at any one sample time and broadening the overall resolution of the pre-concentrator; 3) reactions between the chemical, sorbent, and background matrix can skew measurements by introducing unknowns into the chemical analyzer; 4) the sorbent material is not heated uniformly thus the chemicals will be released at different times and to varying extents.